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Título:
Solvent effect on the styrene oxidation over Co-ZSM-5
Autor/es:
L.B. PIERELLA, S. C. CAGLIERI, C. SAUX,
Reunión:
Conferencia; 9th Latin American Conference on Physical Organic Chemistry; 2007
Resumen:
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The styrene oxidation by hydrogen peroxide (30 wt% in water) over Co-HZSM-5
(0.6 wt% of Co by AA and > 98 % of crystallinity by XRD) has been studied.
The zeolite was obtained by hydrothermal method and the Co incorporation was
performed by ion-exchange. The partial oxidation of styrene was carried out in
a glass batch reactor with a magnetic stirring immersed in a thermostatized bath,
equipped with a reflux condenser, and a thermometer, at 60°C, styrene/H2O2 molar
ratio: 1.7 and 0.2 g
of the catalyst for 7 h. The mains products of reaction were benzaldehyde and
styrene oxide and were analyzed by gas chromatography and mass spectrometry.The
nature of solvent has an important effect on the reaction. In order to
investigate the solvent effect on the styrene conversion, we have used both
aprotic solvents such as: acetonitrile, acetone, ethyl methyl ketone and protic
solvents such as: isopropyl alcohol and secbutyl alcohol. The results reported
in figure show an increase of styrene conversion with the reaction time. The
increase of the activity is more pronounced with the aprotic solvents. This
behaviour may be explained by the increase of substrate concentration in the
hydrophilic pores of Co-HZSM-5.
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Hydrophilic character of Co-HZSM-5, would act forming stable complexes
between alcohols (as solvents) with active sites in detriment of organic
substrate adsorption over the surface of the zeolite, decreasing the activity
for the reaction. New Brönsted acid sites could be generated by the protic
molecules adsorption over Lewis acid sites of the Co-HZSM-5 samples, decreasing
the activity for the reaction. Acetonitrile (aprotic solvent with the highest
dielectric constant showed in the table) reveals to be the best solvent in term
of activity for the reaction. This behaviour could be associated with the
poisoning of Brönsted acid sites by the basic acetonitrile molecules. High
benzaldehyde selectivity (> 80 mol%) and low styrene oxide selectivity (<
10 mol%) could be obtained in all the
experiments.